Need help! how do i make prefix numbers for credit cards mastercard and visa validated using javascript?

Question

How do i make prefix numbers for credit cards mastercard and visa validated using javascript? im struggling to find a solution.

<script type="text/javaScript">

function validate(orderform){

if (document.orderform.CreditCardNumber.value.length < 16) { //Checking the card number is 16 digits

alert("Sorry, You need to enter Your Credit Card Number. It must be 16 digits!")

return false

this is all i have

Answer 1

this is my html for credit card type and number

<select name= "card" id="cardtype">

    <option value="0" selected>Select Credit Card</option>

  <option value="Mastercard">Mastercard</option>

  <option value="Visa">Visa</option>

</select></p>

<input type="text" id="credit" name="CreditCardNumber" placeholder="Your Card Number..." size="16" maxlength="16">

Answer 2

Try this function

function checkNum(numString) {
var re= new RegExp(/^\d{16}$/);
if(!numString.match(re)) {
console.log("16 numbers are required");
return false
}
console.log("16 numbers entered");
return true;

}

checkNum(document.orderForm.creditCardNumber.value);